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Suppose that X is Banach space. How to prove that the center of algebra B(X)( bounded operators on X) consists only of operators aI, where a is scalar and I is identity operator?

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Maybe this is hw ? Anyway, you always have rank one operators $x\mapsto L(x)v$, $v\in X$, $L\in X^*$, and any operator which commutes with all of them is easily seen to be scalar.

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Thank you very much BS! –  ivo Mar 17 '11 at 16:11
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The argument of BS works also in the case where $X$ is a Hausdorff locally convex space since the topological dual still separates points (by Hahn-Banach). This is enough to show that the (continuous) finite rank operators act transitively on non-zero vectors from which it follows that their center is already trivial. But then all bounded operators have at most the center of the finite rank ones and the multiples of the identity.

So if you want to go beyond trivial center you have to consider topological vector spaces which are not lc, pretty weird ones ;) I don't know the answer for $L^p$ spaces with $p < 1$...?

I guess the more interesting problem is whether there are more bounded operators than the continuous finite rank operators and the multiples of the identity...

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@Stefan: There are subspaces of $L_p$, $p<1$, on which every continuous linear operator is a multiple of the identity. See MR0617557 (82j:46039) Kalton, N. J.; Roberts, James W. A rigid subspace of $L_{0}$L0. Trans. Amer. Math. Soc. 266 (1981), no. 2, 645–654. 46E30 (46A22) –  Bill Johnson Mar 17 '11 at 15:42
    
@Bill: Thanks for the reference! –  Stefan Waldmann Mar 17 '11 at 16:05
    
@Stefan : On a Banach space you have at least the operators $\lambda I + N$ with $N$ nuclear, i.e. the sum of an (operator-norm) absolutely convergent series of rank one operators. According to Gowers, it is not known if this can be all of the bounded operators, but Argyros and Haydon have constructed a space with all operators scalar + compact. See gowers.wordpress.com/2009/02/07/… for this interesting story. –  BS. Mar 17 '11 at 16:10
    
@BS: Thanks also for that reference. Interesting indeed. Yeah, of course you can still complete the finite rank ones, so my above statement is sort of stupid :) –  Stefan Waldmann Mar 17 '11 at 16:12
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Take rigid $X$ and $Y$ s.t. the only operator from $X$ to $Y$ is zero and similarly from $Y$ to $X$. Then the center of $B(X \oplus Y)$ is itself and is two dimensional. Unfortunately, Kalton is no longer with us to prove that such a pair of spaces exists... –  Bill Johnson Mar 17 '11 at 16:33
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If I see it correctly, then take a Hamel basis $(v_i)$ with $i \in I$ of the Banach space $X$. Then the coordinate switch $T_{i,j}:v_k \mapsto \delta_{i,k}v_j$ is bounded, because finite dimensional subspaces have a complement in a Banach space. Writing down any $S$ in the centre of $\mathscr{B}(X)$ as a matrix with rows and columns indexed by $I$, commutativity of $S$ and $T_{i,i}$ shows that is must be diagonal and commutativity of $S$ and $T_{i,j}$ shows that all diagonal entries must be the same. Hence $S$ is a scalar multiple of the identity.

The essence of the argument is that finite dimensional subspaces are complemented in Banach spaces.

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